Set up ReadTheDocs documentation, CI for RTD with Travis

[jwflory]

This PR does three things:

• Moves contributing guidelines from the wiki into a CONTRIBUTING.md file
  • Why?: Better leverages GitHub's UI/UX for placing contrib. guidelines in this location
• Initializes a ReadTheDocs environment (see it here)
  • Why?: Moving towards a more VCS-friendly docs setup, also better integrating docs across multiple repos (since we have some sprawl in our projects)
• Enable TravisCI support for building docs
  • Why?: Will use TravisCI to make sure our docs are still able to build and error out if not